As the number of users of telecommunications networks increases, so there is an ever increasing demand for the elements of those telecommunications networks to be able to handle more traffic. This has led to the development of concentration and signalling techniques such as those defined in protocols such as TR303 or V5.2.
One area in which it is contemplated to use such concentrated interfaces is that of wireless telecommunications systems, where, as the number of subscribers to the wireless telecommunications system increases, so there is an increasing demand for the air interface resources to handle more user traffic.
One particular wireless telecommunications system which has been proposed involves dividing a geographical area into cells, each cell having one or more central terminals (CTs) for communicating over wireless links with a number of subscriber terminals (STs) in the cell. These wireless links are typically established over predetermined frequency channels, a frequency channel typically consisting of one frequency for uplink signals from a subscriber terminal to the central terminal, and another frequency for downlink signals from the central terminal to the subscriber terminal.
The system finds a wide variety of possible applications, for example in rural, remote, or sparsely populated areas where the cost of laying permanent wire or optical networks would be too expensive, in heavily built-up areas where conventional wired systems are at full capacity or the cost of laying such systems would involve too much interruption to the existing infrastructure or be too expensive, and so on.
The central terminal is typically connected to a telephone network and exists to relay messages from subscriber terminals in the cell controlled by the central terminal to the telephone network, and vice versa. By this approach, an item of telecommunications equipment connected to a subscriber terminal may make an outgoing call to the telephone network, and may receive incoming calls from the telephone network.
Due to bandwidth constraints, it is not practical for each individual subscriber terminal to have its own dedicated frequency channel for communicating with a central terminal. Hence, techniques have been developed to enable a number of different ST-CT communications to be handled simultaneously on the same frequency channel without interfering with each other. One such technique involves the use of a "Code Division Multiple Access" (CDMA) technique whereby a set of orthogonal codes may be applied to the data to be transmitted on a particular frequency channel, data relating to different ST-CT communications being combined with different orthogonal codes from the set. Signals to which an orthogonal code has been applied can be considered as being transmitted over a corresponding orthogonal channel within a particular frequency channel.
One way of operating such a wireless telecommunications system is in a fixed assignment mode, where a particular ST is directly associated with a particular orthogonal channel of a particular frequency channel. Calls to and from items of telecommunications equipment connected to that ST will always be handled via that orthogonal channel on that particular frequency channel. That orthogonal channel is always available/dedicated to that particular ST.
However, to increase the number of users that may be supported by a single central terminal, an alternative way of operating such a wireless telecommunications system is in a Demand Assignment mode, in which a larger number of STs are associated with the central terminal than the number of traffic bearing orthogonal channels available. These orthogonal channels are then assigned to particular STs on demand as needed. This approach means that far more STs can be supported by a single central terminal than is possible in a fixed assignment mode, the exact number supported depending on the level of dial tone service that the service provider desires.
However, the use of a demand assignment mode complicates the interface between the central terminal and the switch of a public switched telephone network (PSTN). On the switch side interface, the CT must provide services to the switch as though all of the subscribers are connected with direct service even though they may not be actually acquired to a radio frequency channel. Regardless of whether the ST is acquired or not to the switch, all the subscribers must have a presence at the interface to the switch. In a typical fixed assignment mode of operation, it has been known to support 60 subscriber terminals (with two lines each) from a single CT, the CT having 4.times.2 Mbit (4.times.30 channels) digital interfaces to the switch. There is a fixed relationship between end user service and the channel of a 2 Mbit interface to the switch in such an embodiment. Given that, through the use of demand assignment, the number of subscribers could run into the hundreds or thousands, then, without some form of concentration, it is clear that a large number of interfaces to the switch would need to be provided. However, most PSTN switches still use unconcentrated interfaces such as V5.1 or CAS, and only relatively few use concentrated interfaces such as TR303 or V5.2.
Hence, it would be desirable to be able to operate such a wireless telecommunications system in a demand assignment mode of operation in order to increase the number of users that can be supported, but without having to provide the large number of interfaces to the switch that most switches with unconcentrated interfaces will require.